This invention relates to the chemical vapor deposition of diamond, and more particularly to the use of an improved gaseous mixture therein to increase growth rate and modify crystal habit and growth orientation.
The fabrication of diamond by chemical vapor deposition (hereinafter sometimes "CVD") is well known it typically involves activation of a mixture of hydrogen and hydrocarbon gases at low pressure in proximity to a substrate, resulting in the creation of elemental carbon species which deposit on the substrate as diamond. Thermal activation may be by radiation (e.g., microwaves), by means of a heated filament or both.
The use of gas mixtures containing other gases is also known. For example, Beckmann et al. in Diamond and Related Materials, 1, 164-167 (1992), describe the addition of oxygen in amounts apparently in the oxygen/carbon mole ratio range of 0.7-1.25, although the precise values are uncertain since the ratio is sometimes identified as O/C and sometimes as C/O. Maximum diamond growth rate appears to be attained at an oxygen/carbon mole ratio of 0.66.
European patent application 376,694 describes a CVD method for deposition of diamond which employs a mixture of (A) hydrogen, (B) an inert gas, (C) a carbon atom-containing gas, typically an aliphatic hydrocarbon, and (D) an oxygen atom-containing inorganic gas, typically oxygen. The gas proportions in the mixture are defined essentially in terms of mole percentages of inert gas and hydrocarbon and mole ratio of oxygen to hydrocarbon, the ranges of all of said proportions being very broad. The advantages of the disclosed method are alleged to be high transparency of the diamond produced, a low defect rate, high strength and increased growth speed.
It has been found, however, that serious problems are encountered at certain ratios within the ranges designated in the European patent application. Oxygen proportions above certain threshold values, for example, cause partial oxidation of the carbon deposited on the substrate. High nitrogen proportions decrease the hardness of the product, suggesting some unknown variety of defect formation.
Diamond produced by the CVD method normally has a &lt;110&gt; growth orientation with (111) growth facets. This crystal structure is in part a result of to the optimum substrate temperature in the range of 600.degree.-850.degree. C. which also has the effect of reducing internal stresses and cracking. At higher temperatures, especially above 930.degree. C., the diamond is deposited in the form of cubic crystals with a &lt;100&gt; growth orientation and (100) facets, but it has poor thermal conductivity and a high degree of internal stress.
It would be very advantageous, if the aforementioned problems could be solved, to grow diamond in the &lt;100&gt; direction and with (100) facets, since stacking faults with such cubic crystals are minimized and the grains are substantially defect-free. It would also be of interest to increase the growth rate of high quality CVD diamond.